Methamphetamine dependence is a significant public-health concern. The results of clinical trials suggest that agonist replacement therapies (e.g., d-amphetamine) may be effective for methamphetamine dependence. The present project has three specific aims. The first specific aim is to demonstrate the safety, tolerability and behavioral effects of intranasal methamphetamine in humans maintained on d-amphetamine. To accomplish this aim, we will conduct one experiment in which non-treatment seeking volunteers with recent histories of illicit stimulant use will receive ascending doses of intranasal methamphetamine while maintained on increasing doses of d-amphetamine (Exp. 1). Cardiovascular indices will be used to determine the safety and tolerability of the d-amphetamine-methamphetamine combinations while subjective-effect questionnaires will be used to characterize the behavioral effects. The results of this experiment will guide the selection of doses to be tested in subsequent studies. The second specific aim is to demonstrate that d-amphetamine maintenance attenuates the reinforcing effects of methamphetamine. To accomplish this aim, we will determine the reinforcing effects of intranasal methamphetamine during d-amphetamine maintenance using a progressive-ratio procedure (Exp. 2). Pharmacotherapies that attenuate the reinforcing effects of methamphetamine may be effective for initiating abstinence. The third specific aim is to demonstrate that d-amphetamine maintenance attenuates the discriminative-stimulus effects of methamphetamine. To accomplish this aim, volunteers will learn to discriminate intranasal methamphetamine (Exp. 3). The discriminative effects of methamphetamine may be involved in relapse to drug-taking behavior in that an initial dose (i.e., a lapse) may function as a discriminative stimulus signaling the availability of more drug. Pharmacotherapies that attenuate the discriminative-stimulus effects of methamphetamine may be effective for preventing relapse. The proposed research will provide important additional clinical information regarding the efficacy of agonist replacement therapies for methamphetamine dependence. By using two sophisticated human laboratory procedures, one to model abstinence initiation (i.e., drug self-administration) and the other to model relapse prevention (i.e., drug discrimination), the proposed research will determine the mechanisms that mediate the clinical efficacy of d-amphetamine for methamphetamine dependence. By inference, then, the proposed research will identify the optimal conditions under which d-amphetamine would be effective as a pharmacotherapy for methamphetamine dependence (i.e., initiate abstinence or prevent relapse). Finally, because d-amphetamine reduces methamphetamine use, these clinical findings can be used as a reference to determine the predictive validity of human laboratory procedures. Identifying procedures for assessing the efficacy of putative pharmacotherapies via a beside-to-bench or reverse engineering strategy is important because human laboratory studies can be conducted more rapidly and efficiently than clinical trials.